Blue beauty: Queensland sapphires

By Pip Morgan 10 August 2017
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While the majority of sapphires found on the Central Queensland Gemfields are medium to dark blue, some of the most prized finds are the fancy stones for which the area is famous – the yellow, green and multi-coloured sapphires.

EXACTLY WHEN and by whom sapphires were discovered in central Queensland isn’t known. The most popular contender for the title is Archibald John Richardson, a government surveyor who found red zircons in 1873 at Retreat Creek and sent them for testing, believing them to be rubies. At that time, the sapphires accompanying the zircons attracted little interest.

How are sapphires made?

Sapphires are both ancient and very tough. Geologists believe that the sapphires of the Central Queensland Gemfields may have formed between 20-200 million years ago, in deep igneous or metamorphic rocks deficient in silica, such as basalt, which crystallised under high temperatures and pressures

Explosive volcanic basaltic eruptions that took place 15-65 million years ago brought the sapphires — as well as zircons, spinets and garnets — to the surface in a shower of ash. Later, less violent eruptions brought a more normal flow of sapphire-bearing basalt lava to the surface — though the crystals were present in much lower concentrations than delivered by the spectacular volcanic blasts.

Released from their basalt hosts and other volcanic material by weathering and erosion, the sapphires were transported across the landscape by ancient streams and concentrated in various layers of gravel known as wash. In some areas, several wash layers, varying from a few centimetres to over a metre thick, were deposited, inter-layered with sapphire-barren sediments.

Remnants of this older alluvium, called “primary” (high-level) wash, occur above present stream levels, usually on ridges or elevated areas. Further erosion of the primary wash has formed areas of “secondary” (low-level) wash. Modern streams  have also reworked older wash deposits, releasing sapphires into their drainages.

Where do sapphires get their colour?

The mineral corundum, which forms sapphires, is transformed into a kaleidoscopic array of colours by various combinations of trace elements such as titanium, iron, chromium and nickel. The interaction of trace elements in the corundum promote the depth of colour of the gemstone.

For example:

Blue: Ferric iron and titanium

Green: Ferrous iron and/or ferric iron and titanium

Yellow: Ferric iron Orange: Chromium and ferric iron

Violet: Chromium

Pink/red (ruby): Chromium

Colour change

Though rare on the Gemfields, there are sapphires that appear as different colours in daylight and artificial light. These stones, called “colour-change” sapphires, result when lights of different wavelengths are absorbed differently by the stone, causing a variation in the emergent colours.

Star sapphires

In a class by themselves, star sapphires are generally dark blue or green to black in colour and display a dancing six-, or more unusually, 12-pointed star. Such stones contain a mass of fine needle-like mineral inclusions, called “silk”. When cut into cabochons (domed shapes) that reflect light from the inclusions, the star sapphires exhibit the phenomenon that gives them their name.

The sapphire market

It wasn’t until the 1880s that the first commercial production of sapphires took place, though progress over the next nine years was severely hampered by drought and poor demand. The turning point came in 1899 when a display of central Queensland stones at an exhibition in London, coupled with the rising popularity of Australian sapphires among the Russian nobility, led to an upturn in the market. By 1903, sapphire mining was an established industry, centred on two thriving camps —Rubyvale and Sapphire.

A slump during World War I was followed by another brief boom in 1919, which in turn was followed by several decades of limited activity. By 1950, mining had virtually come to a standstill. The early 1970s saw a dramatic change in the industry’s fortunes, with the introduction of machinery mining and the arrival of Thai buyers on the fields.

The price of stone quadrupled overnight as the Thais rushed to buy up large parcels of all grades of sapphire for the heat-treating process that was being developed in Thailand. The age of mechanisation also saw the first conflicts between hand and machine miners. Large-scale machinery tore up the fields, disrupting hand-mining claims, and illegal mining was rife. The good times came to an end in 1977 when other overseas deposits, in particular Sri Lanka, were opened to Thai buyers. So began a pattern of ebb and flow in production that continued.